1. Technical Field of the Invention
The present invention relates to magnetic resonance imaging for an object, and in particular, to both of the magnetic resonance imaging with a highly improved positioning technique for multi-slabs to be located at the positions of object targets, such as a vertebral column, and an easy-to-use interface for the positioning technique.
2. Related Art
Magnetic resonance imaging (MRI) is generalized as a technique based on the behavior of nuclear spins of an object positioned in a static magnetic field. A radio frequency (RF) signal of a Larmor frequency is applied to the object in order to realize magnetic excitement of the object. MR signals are induced and acquired responsively to the excitement, and subjected to reconstruction processing of MR images of the object.
This magnetic resonance imaging is also suitable for imaging of vertebral columns, such as a cervical vertebra, dorsal vertebra and lumbar vertebra. This is because the magnetic resonance imaging allows an object's scanning section to be set at any angle and provides cartilages and others with a higher contrast than that provided by other imaging modalities. Normally, an imaging technique called a multi-slab scan is used for such vertebral columns, providing a plurality of MR images of intervertebral disks.
For example, for diagnosing herniated disks using a magnetic resonance imaging system, a section along the intervertebral disks is scanned based on, normally, a multi-angle and multi-scan technique combined with the multi-slab scan. For this imaging, it is required to plan a scan to determine the positions of slices on a positioning image, which is for instance a sagittal image of the intervertebral disks.
In normal diagnosis, a plurality of slices are located at one or more desired disks in a mutually adjacent and parallel manner in order to examine how deep the hernia develops in the column direction. Such adjacent and parallel plural slices are called a slab. Hence a plurality of slabs are located on the positioning image at arbitrary positions and angles independently of each other. Hence the slabs can be located at different positions at different angles. The determined slabs are then subjected to scanning carried out at a time based on the multi-angle and multi-scan technique.
How to plan an imaging scan, including how to locate slices, which is suitable for imaging the vertebral column of a human body (i.e., an object) is exemplified by for instance Japanese Patent Laid-open Publication Nos. 1994-22933 and 1996-289888.
The former publication discloses, as one aspect, a technique of planning a scan for MR imaging. To be specific, with viewing a sagittal image of a lumbar vertebra, an operator initially positions a slice in parallel with a desired intervertebral disk on the image. Responsively to this positioning operation, a calculator operates according to a series of previously stored procedures so that a single initial slice is placed at a position and an angle, as specified. The calculator automatically places one or more adjacent slices on an upper or lower side of the initial slice.
Referring to FIG. 1, a more practical way of setting a plurality of slabs according to the teaching from the above former publication, which is carried out interactively with an operator, is as follows.
(1) At a predetermined default position on a sagittal image, a first slice is displayed;
(2) a mouse is used to adjust the first slice in its position, thickness, length, angle, and others;
(3) the number of slices to be placed adjacently to in parallel with the first slice is given, thus producing plural slices, thus producing a first slab SL1;
(4) like the above, a second slice is displayed at a desired disk location;
(5) a mouse is used to adjust the second slice in its position, thickness, length, angle, and others; and
(6) the number of slices to be placed adjacently to in parallel with the second slice is given, thus producing plural slices, thus producing a second slab SL2.
Through these operations, as exemplified in FIG. 1, the first slab SL1 consisting of three slices and the second slab SL2 consisting of two slices are designated.
Meanwhile, of the foregoing publications, the latter discloses a technique of planning imaging carried out by an X-ray tomographic radiographing apparatus. Such a technique can also be performed by a magnetic resonance imaging system. Practically, an auxiliary image (X-ray spectroscopic image) acquired around a vertebral column is used to recognize each intervertebral disk, and then specify a middle position in a plane between intervertebral disks to determine a scan position. Further, from the auxiliary image, a centerline that passes through the vertebral column is drawn to detect a scan angle as being perpendicular to the centerline. The thus-detected scan position and scan angle are incorporated in the planning data.
Normally, the vertebral column in the human body is curved three-dimensionally, so that planes positioned in parallel with the intervertebral disks are directed in various ways, respectively. The conventional scan plan techniques use a single two-dimensional image, as described by the foregoing publications. Thus it was very difficult that each slice to be imaged was almost completely in accord with each intervertebral disk.
When taking these conditions into account, to precisely determine the directions of slices in parallel with individual intervertebral disks tilted three-dimensionally may result in a second technique of using a plurality of images to determine a scan position and a scan direction. The use of the plurality of two-dimensional images means that a scan plan is established on each of the images. A period of time required for the planning is therefore made longer remarkably, thus making the total imaging time longer as well. Adopting the second technique is not practical.
Even for the same vertebral column, degrees of diagnostic interest are dependent on individual intervertebral disks. It is desired that the number of slices assigned to each intervertebral disk be therefore freely changeable every intervertebral disk, according to an upper limit of the number of slices, which results from a pulse sequence to be used, and degrees of medical interest. However, it was difficult to accurately change the number of slices every intervertebral disk on each two-dimensional image. In addition, using a plurality of two-dimensional images for changing the number of slices was also almost impossible when considering a time limit.
Moreover, the foregoing technique for positioning slices (slabs) causes an inconvenience when an operator desires to change or adjust contents of the parameters of the slabs that have been determined once. Such cases happen when the number of slices incorporated in a slab is desired to be changed or sizes (e.g., width and/or length) of a slice are desired to be changed. If assuming that changes in the number of slices composing a slab is desired, the following operations will be carried out interactively with an operator:
(1) a mouse is used to move a cursor to a desired slab on a positioning image for selection thereof;
(2) the mouse is again used to move the cursor to a window in which the number of slices are inputted, the window being positioned outside the positioning image, and to select the window; and
(3) a keyboard is used to input a desired numeric value for the number of slices.
As understood from the above, it is required for an operator that both the mouse and the keyboard be used to change values of any parameters of slices (such as the number of slices, sizes of a slice, or others). Using both tools is thus very burdensome for the operator. In addition, whatever the operator wants to change parameters of a slice, the operator should move the cursor between the positioning image and the parameter-changing window, thus amplifying the burdensome operations.
Differently from setting slices to be scanned, a further region to suppress influence resulting from blood flow and others, called a saturation region, should frequently be determined on a positioning image. If one or more saturation regions are determined, an RF saturation pulse is previously applied to the saturation regions, so that signals acquired from the regions are suppressed.
In cases where setting such saturation regions is desired, a saturation region of an arbitrary angle and size is placed at a default position on a positioning image in the similar manner to that for slices. Then a mouse is used to click part of the initial saturation region to move and/or rotate it, thereby being adjusted into a final saturation region of a suitable angle and size.
However, when the number of saturation regions increases, the operational work becomes heavy in proportional to the number of saturation regions and a time required for the setting work is made longer, reducing efficiency thereof.